1. Field of Invention
The present invention relates to an electric device. More particularly, the present invention relates to an electric device that can adjust the output voltage according to the loading conditions.
2. Description of Related Art
Portable devices, such as the digital camera, notebook computer, personal digital assistant (PDA), and cellular phone, have been developed so abundantly that they have become the dominant trend of electric products. The battery, power adapter and DC-DC converter are the essential parts of these portable devices.
The Buck DC-DC converter is 80% more efficient than the traditional linear converter. However, the DC-DC converter has lower efficiency under higher input voltage and has higher efficiency under lower input voltage.
FIG. 1 is a functional block diagram of a traditional power supply system. The adapter 105 generates a fixed voltage for charging the circuit (charger) 130 to charge the battery 110, and this fixed voltage also supplies the power for the system 115 via a first switch (SW1) 120 and couples of DC-DC converters. When the system 115 is not coupled with the adapter 105, the second switch (SW2) 125 is conductive and the battery 110 supplies the power for the system 115.
Because the supply voltage has to be higher than the whole voltage of the battery set, the adapter 105 needs to output higher voltage when charging. However, if the adapter 105 always outputs higher and fixed voltage whether charging or not, it causes the adapter to be less efficient. Moreover, the charging time is just a fraction of the whole operation time for a notebook computer; thus the efficiency of adapter 105 is made even less efficient by outputting high voltage continuously.